Beam tube receiver input



Sept. 6, 1949. J- RMOORE 2,480,872

BEAM TUBE RECEIVER INPUT Filed Nov. 28, 1942 no |l H6 5 I4 l2 L TRANSMITTER l SIGNAL LIMITER AND RECEIVER 6/ KEYER CONVERTER l I I 2| I LOCAL l MASTER OSCILLATOR i OSCILLATOR a l l TRANSMITTER I30 oscnumoa '27 g {1 FBG. 2 E T i TJEFLECTEON VOLTAGE A TTORNE Patented Sept. 6, 1949 BEAM TUBE'TRECEIVEK INPUT James :R. Moore, .Rumson, N: -J.;;assignor 2120 :the I United Statesrof rimericazas mepnesmted' by ..the

Secretaryof War.

Application November 28, mea -sesame. 43752170 (cram-.13).

.( Granted. under "the act of March -B; r883, -as amended-A'pril"30, 1928; 370 O. G. 757) 3 Claims.

The invention described herein maybe manufactured and used bylor .for the Government .for governmental purposes, without payment to me of any. royalty thereon.

This invention relates to radio object locating systems and more particularly to meansifor protecting a receiver in such .systems during. transmitting cycles, the receiver and transmitter being connected toa common antenna.

In the'sys'tems of this kind, powerful pulses of ultra high ffrequency energy are transmitted and radiated by a directional antenna array. If this energy .impinges upon an object capable .of reradiating aradio wave, aportion of this transmitted energy is reradiated, and a small part of this reradiatedenergy, or echo, reaches the same antenna array, It is then impressed .on a suitable receiver which detects this echo signal and produces a visual. indication on .a screen -.of an oscilloscope.

In order to obtain long range with the radar systems, it becomes necessary to mount the antenna, arrays on steel towers as high as practicable. Moreover, to obtain .a, highly directive beam of su'fi'iciently high intensity when transmitting. andsufiic'ien'tly directive receiving .antenna when receiving, the number of .dipole units is increased. This .results in an antenna, array of a relatively .large size, .especially whenthe radio frequency used .is in .the order of 100 megacycles. With these mechanical limitations imposed upon the system, the :use .of a plurality-of independent antenna arrays for sending and receiving .becomes very. undesirable. This is equally-trueof Kor 'X-band radar using a single dipole and a hyperbole reflector. Therefore, it is customary .to usea single antenna for transmitting and .receiv. ing of signals.

The use of a singleantenna, however, requires protection of receiver from ahigh intensity signal impressed 'on a common transmission line during transmitting cycle. If no special means are pro.- vided to isolate-a .sensitive receiver from the .antenna circuit' during .transmitting cycle, the .receiver may be destroyed by high power transmittedsignal.

Certain known methods of protecting radar receivers involve the use .of protective spark gaps connected across the transmission line leading to the input circuit of the receiver. In some instances the spar'kgap arrangement is replaced with diode elements which perform the same protectiveifunc'tion.

The spark gap element ordinarily consists. oi two metal electrodes mountedin a glass envelope filled with an inert ,gassuch as argon, or the like, whichis interposed across a transmission line connecting the receiver .to antenna. If the-transmiss'ioniine represents apart of a tuned circuit, the transmitted-signal forms .as'tanding wave .on the line. .Since'the spark gap element is connected atrthat part of the line where the transmitting voltage-forms a loop, this voltage produces .a discharge in .the .gas .tube, thereby .protecting thereceiver.

'This'mode oi.prote.cting the receiver isnot free of certain disadvantages notable of which are dissipation of .power at the time .of transmission through discharge, and inability to tune properly the transmitter-receiver system. The transmitter channel cannot be kept .rcritically .tuned because the discharge.acrossthetube changes the impedance oflthis channelanddetunesit. If the elements of the transmission channel are so placed .that the transmission channel approaches perfect tuning rat the time of discharge across the spark gaps, then thesame channel becomes detunedjat the (time ofthereception of the echo signals. IIt -.cus'tomar.y, therefore, to operate these systems. so .that they are partially tuned for "transmission as .wellias reception of signals. This. mode -.-of :operation obviously limits r efiiciency of such system.

Tllheaboveediificulty may-be minimized by following ltheteachings of -my objection. According to the invention aspecial cathode ray beam tubeis used insuchamanneras to act as a converterias well as .an effective protector of receiver whenfthe signals from transmitter are-impressed onlthe receivenchannel.

It is, therefore, an .object of my invention to providean improved-form of protective circuit for a receiver connected .tothe-same antenna as the transmitter.

Aiurther object of my invention is to combine in a single. element. .an. improved form .of a. .protective circuit 'withan improved form of an-electron beam. device particularly useful for frequency conversion .in @superheterodyne reception.

Another obiectofmy-invention is to provideran improvediorm .oi protective system which ena'blesmore Taccura'tetuning .of .transmitting and receiving .channelswith the resulting higher efiiciences .during transmitting and receiving cycles.

'Thetnovellfeatures which I believe to becharacteristicnf myinvention are set forth with particularityin 'theappended claims, :but the invention itself will best be understood by reference to the ,followingdescription .taken .in connection withithe accompanying drawings in which;

Fig. 1 illustrates a block diagram of a pulse nected in the receiver channel;

Fig. 2 illustrates a schematic diagram of the protective signal limiter; and

Fig. 3 illustrates an anode current curve of the signal limiter.

Referring to Fig. 1, a common antenna I is connected to a transmitter I 4 and a receiver I5 by transmission lines II, I2, and I3. Transmitter I 4 is keyed by a keyer I6, this keyer being connected to a master oscillator II. Receiver I5 comprises a signal limiter and converter I8 connected to transmission line I3, a local oscillator I9, the intermediate and audio frequency stages of receiver 20, the output of which is connected to an oscilloscope 2I whose sweep circuit is con trolled by master oscillator II.

The operation of the pulse echo system illustratedin Fig. l, is as follows: Keyer It modifies the sinusoidal wave impressed upon it by master oscillator II into a periodic series of peaked pulses of short duration. These pulses are utilized for generating equally short pulses by transmitter I 5-, which are in turn radiated by a highly directional antenna II). If there are any echoes they are received by the same antenna during the idle periods of transmitter I 4. The echoes as well as the transmitter pulses are impressed on the signal limiter and converter I8 where they are combined with the frequency of local oscillator I 9 and converted into pulses of intermediate frequency. The I. F. pulses are impressed on receiver 23, which converts them into video pulses and impresses the latter on the deflection plates of oscilloscope 2! whose sweep circuit is under control of master oscillator I I. The visual indications produced on the oscilloscope are utilized for determining the location of the object producting the echoes.

Since my invention resides in protecting receiver 20 by means of the signal limiter and converter I8, the schematic diagram of the latter is illustrated in Fig. 2. Referring to Fig. 2, antenna IIJ, transmission lines II and I2, and transmitter I4, illustrated in Fig. 1, are also illustrated in Fig. 2. The signal limiter and converter I8, illustrated in schematic form in this figure, comprises a beam tube described in the U. S. Patent to Edward W. Herold, No. 2,294,659, dated September l, 1942, whose disclosure is hereby incorporated as a part of this specification.

This tube comprises a cathode IBI, a plurality of accelerating and focusing electrodes I22, I03, and Hi l, deflection plates I85, I06, and an anode IGl. In the center of an aperture in electrode I64, there is a rod I98 electrically connected to electrode I04. The operating characteristics of this tube are fully described in the above mentioned patent to Herold and need not be repeated here. The deflection plates Illa-496 of this tube are connected to a quarter wave length, or odd number of quarter wave lengths, trombone I09 whose electrical length is adjusted at the points of contact of a condenser I I ll with the conductors forming this trombone. The trombone I29 is connected to transmission line I2 through conductors III--II2 and condensers II3II I. The conductors IIII I2, which correspond to line I3 illustrated in Fig. l, are provided with adjustable links I l5I I '6 which are so adjusted that the electrical length of the connection between trombone IE9 and transmission line I2 is equal to a half-wave length or a multiple of it, the wave length being that of transmitter I4. Moreover, these conductors form a transmission line which is to be '4 matched to the main transmission line as well as to the trombone.

Local oscillator I 9 is inductively or capacitively coupled to trombone I09, an inductive coupling loop I20 being illustrated inthe figure. The output of converter I8 is connected to an intermediate stage amplifier VI through a condenser I I! and a coil IIB whose inductance and distributed capacitance act as a tuned circuit for the intermediate frequency signal. A source of direct current po tential I2I and a potentiometer II9 are used for adjusting the normal position of the beam so that it strikes rod I 08 when there are no signals impressed on the converter, and the normal plate current is at a minimum. Source I 2| and potentiometer I I9 are connected to the deflection plates I06 and I05 respectively through high resistances I22 and I23 and two conductors constituting the u. h. f. transformer I09. By adjusting potentiometer I I9 it is possible either to decrease or increase the voltage impressed on plate I05 as compared to the voltage impressed on plate I06 with the concomitant shifting of the electron beam toward the plate having higher positive voltage. The accelerating and focusing electrodes are connected to a source of potential I25 over a conductor I26. The same source furnishes positive potential for plate IQ'I, which is connected to a voltage divider I27 through a plate resistor I228. The accelerating and focusing electrodes and rod I08 are all shunted to ground through a filter condenser I30 so that the u. h. f. signal does not appear in the voltage divider and source I25. A filtering condenser I32 is also connected across the upper portion of potentiometer I21 so that the u. h. f. signal appearing across resistance I28 is shunted to ground through the condensers I32 and I30.

The operation of the system is as follows: when transmitter I4 is transmitting, large power u. h. f. pulses are impressed on the electrically fiat transmission line I2 and on the half-wave length link I l I-I I2. The voltage of pulses is further amplified by quarter wave length trombone H19 which acts as a step-up transformer. Since the high voltage end of the transformer is connected, across the deflection plates I05-I06, the electron the beam of converter I8 will be oscillated at the u. h. f. frequency of the transmitter modulated by oscillator I9, 1. e. at intermediate frequency. Since the maximum deflection positions the beam will be deflected beyond the aperture openings in electrode IIM, no space current will reach plate Iii! and the signal impressed on tube VI at this instant will be equal to zero. One can very readily see that this is the case by referring to the deflection voltage-anode current curve for this tube illustrated in Fig. 3. At point a the beam is centered on rod I 03, and the magnitude of the anode current depends upon the focusing of the beam, the width of rod I08, and the amount of stray electrons reaching anode [61. When the beam is deflected to the right in Fig. 3, the anode current reaches a maximum at point 1), which takes place when the beam reaches plate I 97 through either aperture, and then decreases when part of the beam begins to strike electrode IE4. When the entire beam is deflected toward electrode Ill l, the anode current is equal to zero, which corresponds to point e on the curve. The same is true when the beam is deflected to the left. The maximum cathode-anode current delivered by the converter is sufliciently low so as to prevent any injury of the receiver during sending cycle of transmitter I4. When an echo is received, it is impressed on trombone I 09 in exactly the same manner as the transmitted signal, transformer log steps up the voltage, and since the echo signal is ordinarily very weak, converter It] now acts as an ordinary full-wave converter, as described in the above mentioned patent to Herold. Comparison of my system with the prior art discloses several important advantages. Converter l8 can be connected to the common transmission line l2 through the voltage-amplifying trombone I09 since the impedance across the deflection plates 105-406 is very high. This results in an amplification of the weak echo signals and a higher signal to noise ratio in the output of the receiver. Since the radio frequency of the echoes is the same as the radio frequency of the transmitted signal producing them, the transmitted signal will be also amplified by transformer Ill-9. However, this high voltage transmitting pluse will not impair the receiver since during the transmitting cycle the beam tube acts not only as a converter but also as a limiter. These desirable transmitting-receiving operating characteristics are obtained without imposing undue shunting load on the main transmission line l2 during the transmitting cycle, which results in the delivery of high percentage of transmitter power to the antenna for transmitting high power exploratory pulse.

From the foregoing description of my invention it will be clear to those skilled in the art that there are numerous extensions and modifications within the scope thereof. Thus the invention should not be regarded as limited to the specific embodiment shown, but is rather defined by the scope of the appended claims.

Iclaim: 1. In a radio-pulse echo object locating system having a transmitter and a receiver connected to a common transmitting-receiving antenna, said receiver being connectible to said antenna through a constant intensity electron beam tube having beam deflecting means, two apertures in a grounded electrode above and below the normal position of said beam, and an anode in the electron-intercepting relationship with respect to said apertures, said anode being connected to said receiver, the method of protecting the receiver from overloading during the transmitting cycle which includes the steps of impressing an ultra-high frequency transmitting pulse on said antenna and on said means, generating four, spaced, unipolar pulses in response to each cycle of said ultra-high frequency at said anode by deflecting said beam beyond the boundary of one aperture at a positive half cycle of said frequency, and beyond the boundary of the other aperture at a negative half cycle of said frequency, and generating two, spaced, unipolar pulses at said anode in response to echoes of said pulse.

2. A radio system including an antenna, a transmitter and a receiver connected to said antenna, an electron beam tube in the input circuit of said receiver, said electron beam tube having beam deflecting means connected to said antenna and said transmitter, an accelerating and focus sing electrode having two apertures, said electrode being grounded through a condenser, and an anode connected to said receiver and in beamintercepting relationship with respect to said apertures, said deflecting means deflecting said beam in response to a pulse generated by said transmitter so as to ground said beam through said grounded electrode during the larger portion of said pulse, whereby only relatively weak pulses are generated by said beam in said anode, and impressed by said anode on said receiver, when said bealm reaches said anode through said apertures.

3. In a radio pulse-echo system, a transmitting-receiving antenna, a transmitter, a transmission line interconnecting said transmitter and said antenna, a receiver, an electron beam tube at the input of said receiver, said tube having a cathode, electron accelerating and focussing electrodes, apertures in said electrodes, an anode in electron-intercepting relationship with the apertures adjacent to said anode, deflection plates in said tube for oscillating said beam across the apertures adjacent to said anode, a connection between said anode and said receiver, a low impedance path between said electrodes and said cathode, and connnections between said transmission line and said deflection plates, said transmitter, when transmitting, oscillating said beam across said apertures and against said electrodes whereby said tube acts as a limiter during transmitting ,cycles of said system.

JAMES R. MOORE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,035,958 Girardeau Aug. 20, 1912 2,277,731 Toth Mar. 31, 1942 2,287,296 Dallos June 23, 1942 Certificate of Correction Patent N 0. 2,480,872 September 6, 1949 Column 2, line 30, for the word objection read invention; column 3, line 39,

for producting read producing; column 4, line 51, after Since insert in; column 6, line 31, for eccelerating read accelerating;

and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 25th day of April, A. D. 1950.

THOMAS F. MURPHY,

Assistant Oommzlesz'aner of Patents. 

